In circuit boards, such as printed circuit boards (PCBs), electrically-conductive traces are used to interface electrical contacts of ICs to electrical contacts of other ICs to allow the ICs to transmit electrical signals among them. In such environments, there is at least one transmitting (Tx) IC and at least one receiving (Rx) IC. The Tx IC includes at least one Tx driver circuit that drives the respective trace to transmit an electrical signal over the trace. The Rx IC includes at least one receiver circuit that receives the electrical signal transmitted over the trace. Typically, the transmitter driver circuit includes a differential transistor pair that generates a differential signal that is transmitted over two side-by-side traces of the PCB. The Rx circuit includes decoding circuitry that decodes the transmitted differential signal into a logic 1 or logic 0.
It is not uncommon for the Tx and Rx ICs to operate at different supply voltages. For example, high performance, high signal integrity ICs typically operate at 2.5 volts (V) and are designed with current mode logic (CML) created via silicon germanium (SiGe) processes to achieve better input sensitivities at the receiver side, higher output swing of the differential signal on the transmitter side, and lower jitter on retimer circuits. On the other hand, the Rx ICs are often application specific integrated circuits (ASICs) or digital signal processor (DSP) ICs that are designed in complementary metal oxide semiconductor (CMOS) processes to have reduced geometries that provide benefits in terms of power and area savings. Often times, deep submicron CMOS processes are used to make these ICs, and the resulting ICs operate at supply voltage levels of 1.0 V or less.
Alternating current (AC) coupling capacitors are typically disposed in between the transmitter and receiver ICs and connected to the traces. The AC coupling capacitors decouple the direct current (DC) components of the two ICs.
FIG. 1 illustrates a block diagram of a Tx IC 2 and an Rx IC 3 mounted on a PCB 20 and interfaced via two PCB traces 4 and 5 having respective AC coupling capacitors 6 and 7 connected in line in the respective traces 4 and 5. The Tx IC 2 includes a Tx driver circuit 8 that includes a Tx analog front end analog (AFE) portion 9, a differential transistor pair 11, a current source 12, two resistors 13 and 14, a 2.5 V voltage supply 15, and two output terminals 16 and 17. The AFE portion 9 includes analog circuitry (not shown) that drives the bases of two bipolar junction transistors (BJTs) 21 and 22 of the differential transistor pair 11. The emitters of the BJTs 21 and 22 are connected to one another and to the current source 12, which is connected to ground. The collectors of the BJTs 21 and 22 are connected to the output terminals 16 and 17, respectively, and to first terminals of the resistors 13 and 14, respectively. Second terminals of the resistors 13 and 14 are connected to the 2.5 V voltage supply 15. The output terminals 16 and 17 of the Tx driver circuit 8 are connected on first ends to contacts 31 and 32 of the PCB 20 on which the Tx and Rx ICs 2 and 3 are mounted and are connected on second ends to the first terminals of the resistors 13 and 14, respectively. The contacts 31 and 32 of the PCB 20 are connected to first ends of the traces 4 and 5, respectively.
The Rx IC 3 includes a receiver circuit 33, which includes an Rx AFE portion 34, first and second resistors 35 and 36, first and second ESD diodes 37 and 38, a first voltage supply 41, a second voltage supply 42, a first input terminal 43 and a second input terminal 44. The voltage supplies 41 and 42 typically provide supply voltages that are equal to or slightly less than 1.0 V. The input terminals 43 and 44 are connected on first ends to the Rx AFE portion 34 and on second ends to contacts 46 and 47, respectively, of the PCB 20. The contacts 46 and 47 of the PCB 20 are connected to second ends of the traces 4 and 5, respectively. The anodes of the ESD diodes 37 and 38 are connected to the input terminals 43 and 44, respectively. The cathodes of the ESD diodes 37 and 38 are connected to the voltage supply 41. First terminals of the resistors 35 and 36 are connected to the input terminals 43 and 44, respectively, and second terminals of the resistors 35 and 36 are connected to the voltage supply 42.
The AC coupling capacitors 6 and 7 are connected in line along the traces 4 and 5, respectively, such that capacitor 6 is interposed in between PCB contacts 31 and 46 and such that capacitor 7 is interposed in between PCB contacts 32 and 47. The resistors 13, 14, 35, and 36 are typically 50 ohms each. During operation, the Tx AFE portion 9 drives the differential transistor pair 11 to cause the differential pair 11 to produce a differential voltage signal across the output terminals 16 and 17 of the Tx driver circuit 8. The AC coupling capacitors 6 and 7 decouple the DC components of the Tx and Rx ICs 2 and 3, respectively, and couple the differential voltage signal to the Rx IC 3. The Rx IC 3 receives the differential signal and the AFE portion 34 decodes the differential signal.
Although the placement of the AC coupling capacitors 6 and 7 in line along the traces 4 and 5 provides adequate DC decoupling of the ICs 2 and 3 in cases where the ICs 2 and 3 use the same or different voltage supplies, the inclusion of the AC coupling capacitors 6 and 7 on the PCB 20 increases system costs and complicates the build of high-performance PCB traces, especially when considering that there are typically many such Tx and Rx circuits on a PCB.
The AC coupling capacitors are sometimes incorporated into the Rx IC 3. FIG. 2 illustrates a block diagram of the Tx IC 2 interfaced with the Rx IC 3 via the PCB traces 4 and 5 where the AC coupling capacitors 6 and 7 have been moved to the interior of the Rx IC 3 and connected in line between the input terminals 43 and 44 of the Rx IC 3 and respective inputs of the Rx AFE portion 34. The incorporation of the capacitors 6 and 7 into the receiver IC does not provide DC decoupling in cases where the Tx IC 2 and the Rx IC 3 operate at different voltage supplies, as in FIGS. 1 and 2. In addition, the ESD diodes 37 and 38 may inadvertently turn on if the supply differential voltage between the Tx IC 2 and the Rx IC 3 exceeds the diode chain turn-on threshold voltage. This latter problem can be avoided by stacking ESD diodes to increase the turn-on threshold voltage of the diode chain, but stacking ESD diodes in this manner also degrades the ESD performance of the Rx IC.
Accordingly, a need exists for a way to interface ICs that operate at different supply voltages while providing DC decoupling of the ICs and while preventing inadvertent turn-on of the ESD diodes.